In the related art, in order to realize light weight, high rigidity, and improved smoothness, a hollow member which is formed of aluminum alloy and has a double skin structure (two-face structure) is used in a side structure or a floor structure of a vehicle, an aircraft, or the like, a roof structure of a building, or the like. In addition, as a method of butt-welding metal plate materials (plate-shaped members and portions) such as these hollow members, since a method referred to as friction stir welding has many advantages compared to arc welding (TIG, MIG, or the like) or the like such as an advantage in which welding defects of blow holes, solidification cracking, or the like do not easily occur, friction stir welding is widely used.
In addition, as a method for performing friction stir welding, there is a method referred to as a bobbin tool type friction welding method. In this bobbin tool type friction welding method, as shown in FIG. 4, a rotary shaft 1 and a pin-shaped probe 2 on the tip of the rotary shaft 1 are integrally provided so as to be disposed to be coaxial with each other on an axial line O1, and an upper shoulder (first shoulder) 5 and a lower shoulder (second shoulder) 6 are disposed such that a joint (butting portion) S, which is disposed to butt ends 3a and 4a of a pair of plate materials 3 and 4, is interposed between the upper shoulder and the lower shoulder. In addition, the rotary shaft 1 and the probe 2 are rotated around the axial line O1, and the upper and lower shoulders 5 and 6 rotate at high speed while pressing the joint S between the plate materials 3 and 4 with a predetermined pressing force.
Accordingly, friction heat occurs on a contact surface between the joint S between the pair of plate materials 3 and 4 and the shoulders 5 and 6, and the vicinity of the contact surface is plasticized.
Simultaneously, structures of the ends 3a and 4a (joint S) of the pair of plate materials 3 and 4 intermix with each other due to stirring action according to high-speed rotation of the probe 2, and the ends 3a and 4a of the pair of plate materials 3 and 4 can be strongly welded to each other (for example, refer to PTL 1).